1. Field of the Invention
The present invention concerns a projection type display device that separates light from a light source into the colors red (R), green (G), and blue (B). The device then performs spatial modulation, superimposes the color components again, and projects the resulting light over an element such as a screen.
2. Description of Related Art
Various projection type display systems have been developed in recent years. These systems display color pictures by separating light from a light source into the colors R, G, and B, providing the separated light components with picture image information by performing phase modulation, and then superimposing the components again and projecting them onto an element such as a screen. Highly accurate control over the polarization characteristic of light is demanded in these systems. Furthermore, a polarizing beam splitter; which functions as a polarizer as well as an analyzer, controls the polarization characteristic and its role in the picture quality of projected picture images is tremendous.
Systems using translucent materials that are homogeneous and isotropic with respect to lights and systems in which a translucent material is soaked in a liquid (a liquid soaking type system) and used as the base material have been considered for such a polarizing beam splitter in a projection type display.
Generally, the optical anisotropy that is created within the translucent material induces double refractions due to various causes in a polarizing beam splitter that uses homogeneous and isotropic translucent material. There is an obstruction of the polarization characteristic of the light being disturbed. That obstruction becomes the cause of a reduction in the quenching ratio of the polarization S and the polarization and separation P and causes irregularities in the illumination intensity. The main causes of the optical anisotropy within the translucent material include external stresses generated during processing stages of the glass (cutting, joining with other materials, and formation of a film at the surface, for example) and operations incorporating glass into the optical system (holding with a jig and bonding, for example) thermal stress generated by heating within the glass (adsorption of light energy, for example) or external heating (heating of a peripheral device, for example), and also stress generated when materials that have a different thermal expansion ratio than glass are brought into contact and bonded during heating, for example.
To solve the problem provided by these stresses, many liquid soaking type polarizing beam splitters have been proposed in a structure in which a plate made of a translucent material, which is provided with a coating for a polarizing beam splitter, is soaked in a liquid that is provided with an index adjustment. Such is indicated in U.S. Pat. No. 4,687,301. The index of refraction of the aforementioned liquid is adjusted to be identical to the index of refraction of the aforementioned translucent material. Also, it is not always necessary that the translucent material in plate form be homogeneous and isotropic with respect to light. The reason for soaking the translucent material in a liquid in this manner is to prevent stress from being generated and also to allow coating materials, which could not be used due to a different index of refraction between the interface of the coating and the air, to function as a polarizing beam splitter.
Liquid soaking type polarizing beam splitters, however, also have many problems. Some of these problems are mentioned below.
First, there is a large change in the index of refraction in the liquid resulting from the temperature. Second, there is nonuniformity in the index of refraction resulting from a change in the index of refraction. Third, there is a convection current due to the fact that a liquid is utilized. Fourth, there is a volume change that is also due to the fact that a liquid is utilized. Fifth, impurities, dirt, and bubbles in the liquid produce influences. Sixth, the liquid may leak.
Liquid soaking type polarizing beam splitters basically have provided excellent performance. However, this type of beam splitter also has many problems at the same time. The manufacture of a projection type display device using liquid soaking type polarizing beam splitters, moreover, is extremely time consuming and is linked to an increase in cost. Furthermore, changes in characteristics due to the temperature is a problem that cannot for the most part be avoided.